This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Micro-CT is widely used for small animals imaging in pre-clinical studies. There is significant interest in obtaining not only anatomical but also functional measurements on the same systems. The relatively recent development of functional clinical CT has highlighted the value the fourth dimension (time) in characterizing cardiac function and blood flow. Translation of the technology to preclinical studies has enormous potential to help understand critical pathways in genetic models and to highlight potential concerns in drug safety evaluation. 4D cardiac micro-CT faces important challenges due to the small size of the mouse's heart (the diameter of which is only 5 mm) and rapid heart rates (up to 600 times/minute), thus requiring both high spatial resolution and temporal resolution. Previously, we have implemented a cine cardiac micro-CT based on prospective gating. Prospective cardio-respiratory gating ensures equiangular distribution of projections but involves long acquisition time since each X-ray exposure is triggered when strict conditions are met i.e. at a selected phases of the respiratory and ECG cycles, e.g. end-expiration and the diastole phase in the cardiac cycle. Other groups have performed cardiac micro-CT studies in mice using slip-ring flat panel based CT and retrospective gating. In retrospective gating, the projections are sampled much faster, with an equiangular step but at random time points in the cardiac and breathing cycle. Both ECG and the respiration signals are recorded during acquisition and used post-sampling to cluster the projections into subsets corresponding to different cardiac and respiratory phases. For each of the cardiac phases, the corresponding projections have an irregular angular distribution since many views may be missing. Furthermore, when a large number of cardiac phases are to be reconstructed (e.g. n=10 phases), the number of projections in each subset may be limited also due to restrictions in dosage and scanning time. In this project, a novel sampling strategy named fast prospective gating (FPG) is proposed which combines the advantages of retrospective and prospective gating. FPG can deliver fast scanning similar to retrospective gating but with a regular angular distribution as in prospective gating. To reduce acquisition time and also radiation dose, FPG involves angular under sampling on which we compensate using an ordered subset convex algorithm (OSC) reconstruction algorithm.